Description & Biology

The onion maggot (Delia antiqua) is one of the most destructive insect pest of onions and related plants. Injured seedlings wilt and die. Larger bulbs may survive some injury but are often poor keepers. Once onion maggots infest an area, they seem to be a problem every year. White onion varieties are more susceptible to attack than other varieties.

Onion flies and maggots are very similar to the seed corn maggot. The grayish brown adult is less than a 1/3" and lacks the cabbage maggot’s three dark stripes. These flies will generally stay hidden. When mature, the whitish maggots are about 1/4" long.

The onion maggot overwinters in the pupal stage under debris in or near the soil surface. In about the third week of May, when onions are up or planted, the flies emerge to lay their eggs near the onion plants. In less than a week, the eggs hatch. The maggots feed for two to three weeks. In about two weeks, the second-generation adults emerge from the pupal stage. Cool, wet weather favors development of three generations per year. A complete cycle takes 45 to 65 days.

Stunted or wilting onion plants are the first signs of onion maggot damage. At this time, you may find the maggots in putrid, decomposing onion plants. Light infestations may not kill onions but may make them more susceptible to rots. Onions of all sizes may be attacked, especially in the fall, when cooler weather favors the maggots activity. Damaged onions are not marketable and will rot in storage causing other onions to rot.

Management

Authorities differ as to whether fall plowing is advisable. Some say plowing exposes the pupa to predation and dehydration; others claim it only protects the pests.